Titan: A Vast, Subsurface Ocean?

byPaul GilsteronDecember 14, 2012

Yesterday’s look at a major river on Titan took on a decidedly science fictional cast, but then Titan has always encouraged writers to speculate. Asimov’s “First Law” (1956) tackles a storm on Titan as a way of dealing with the Three Laws of Robotics. Arthur C. Clarke filled Titan with a large human colony in Imperial Earth (1976), and Kim Stanley Robinson used Titanian nitrogen in his books on the terraforming of Mars. As far back as 1935, Stanley G. Weinbaum was writing about a frozen Titan and the struggles of early explorers on that world.

The list could go on, but right now the focus stays on Cassini, which with funding continued through 2017 will continue to give us new and striking discoveries like the river dubbed the moon’s ‘little Nile’ feeding into Ligeia Mare. Nor do I want to ignore the recent work from Howard Zebker (Stanford University) and team, who have been working with Cassini radar data and new gravity measurements to tell us more about the internal structure of Titan and its shape. The idea that Titan boasts a deep subsurface ocean is consistent with the results in this study, which was presented at the annual meeting of the American Geophysical Union earlier this month.

Image: A Cassini view of Titan, with Kraken Mare, a sea of liquid hydrocarbons, visible at the top of the image. The image was taken with the Cassini spacecraft narrow-angle camera on Sept. 14, 2011 using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers. The view was acquired at a distance of approximately 1.2 million miles (1.9 million kilometers) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 26 degrees. Image scale is 7 miles (12 kilometers) per pixel. Credit: NASA/JPL-Caltech/Space Science Institute.

The model of Titan now accepted is that beneath an icy crust perhaps 100 kilometers thick is an ocean of uncertain depth. Below that is a core that is most likely a combination of ice and rock. The ocean is thought to remain liquid thanks to heat generated by the decay of radioactive elements in the core that go back to the earliest days of the Solar System. Zebker’s team has been analyzing how Titan spins on its axis, and specifically how much it resists any changes in that spin, a factor known as the moment of inertia. He explains the result:

“The moment of inertia depends essentially on the thickness of the layers of material within Titan. The picture of Titan that we get has an icy, rocky core with a radius of a little over 2,000 kilometers, an ocean somewhere in the range of 225 to 300 kilometers thick and an ice layer that is 200 kilometers thick.”

That 200 kilometer ice layer doubles the previous estimate and implies there may be less rock in the core and more ice than had previously been estimated. But the work also has to contend with the fact that Titan’s shape is distorted by the gravitational influence of Saturn, flattening it slightly at the poles. And it turns out that Titan’s shape is more distorted than would be accounted for by the basic gravitational model. Working the numbers, Zebker finds that the density of material under the poles would have to be slightly greater than that under the equator.

Reasoning that liquid water is denser than ice, Zebker’s team comes up with this model: Thinner ice at Titan’s poles — about 3000 meters thinner than the average — and thicker ice at the equator, by about the same margin. The researchers used the combination of gravity and topography to suggest that the average thickness of the icy layer over the ocean is 200 kilometers. The variation in internal heat that would account for the changes in surface thickness may well be the result of tidal influences as Titan orbits the gas giant. Zebker again:

“The variation in the shape of the orbit, along with Titan’s slightly distorted shape, means that there is some flexure within the moon as it orbits Saturn. The planet’s other moons also exert some tidal influence on Titan as they all follow their different orbits, but the primary tidal influence is Saturn. The tides move around a little as Titan orbits and if you move anything, you generate a little bit of heat.”

So the internally generated heat that keeps Titan’s ocean liquid comes not only from radioactive elements but tidal interactions between Titan and Saturn as well as between Titan and the planet’s other moons. Putting these factors into play coupled with analysis of the moon’s moment of inertia allows us to piece together a picture of its internal structure. It’s worth noting that as we continue the study of the outer system, we’re now looking at subsurface ocean possibilities in many places, from Triton to Pluto and conceivably even further out into the Kuiper Belt.

The Titan we’re looking at through Cassini is a long way from the Titan Stanley G. Weinbaum imagined in the story I mentioned at the top, which was “Flight on Titan” (Astounding, January 1935). Weinbaum has a Titan that’s more chilly than frigid, one in which the characters flee native lifeforms as they make a forced march over Titanian mountains, all the while carrying the objects of the journey from Earth, the rare gems called ‘flame orchids.’ Weinbaum’s fantasy Titan bears little resemblance to the hazy, hydrocarbon-saturated surface we see, but it’s a testament to the fact that we have always peopled unknown worlds with the objects of our imagination, a compulsion that, given the means, leads inevitably to journeys of discovery.

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billDecember 14, 2012, 16:28

“A Cassini view of Titan, with Kraken Mare, a sea of liquid hydrocarbons, visible at the top of the image.” the white area or dark, Paul ?

It is one of those truth is stranger than fiction; scuba diving the moons of the outer planets.

If you were pitching a script for it back in the 50’s or 60’s they would show you the door. I did see a few movies as a boy about cities under the sea- now it seems they should have been set on those bodies in the outer solar system.

We thought it would be about space suits and the stars overhead; the future of space exploration may be underwater with ice overhead.

Something is very wrong here. Zebker must be truly desperate for a way to fit the models if he postulates that ice is thinner at the poles than the equator. Postulating that this could be a tidal effect seems even more desperate since there could be no way to prevent much more flexing at the equator than at the poles. Perhaps much thicker ice at the poles caused a pole shift a few million years ago?

My extreme suggestion is that there are huge deposits of low density organics at the tropics – after all that is where all those organic dune fields are.

“But“, I here you cry, “the vast majority of organics rains down at the poles“. Ah yes, but all titans life is there, and eats this. I admit that my first suggestion sounds more likely though.

GarryChurch, earlier I noted you were decrying the potential for space storage of propellant. Reading the equilibrium pressure of liquid O2 off a phase diagram gave 2GPa at 200K and about 0.5GPa at 100K, and I could see your problem. Now you seem to advocate a real Journey ot the Centre of [Titan] for James Cameron types as the real final frontier. The pressure withstood here must be at least 0.3-0.4GPa. This might only be about three times Challenger deep, but I wonder if you are being a little inconsistent (perhaps compression resistance presents an easier engineering task??)

I did not research the pressure in these possible liquid oceans. But if that is accurate it is certainly a daunting proposition to explore them with human crewed submarines. Are you taking into account the lower gravity on Titan? Tremendous pressure would mean transporting a titanium sphere several feet thick there. Not an incredible solution if you make it out of Lunar Titanium and Lunar Solar Power.
After reading Ozzie Zhener’s Green Illusions and the Dan Criswell Lunar Solar Power concept web entries I am becoming skeptical about any other possible approach having any relevance in my lifetime. I turned fifty recently and though they sent John Glenn on a mission, I am not confident of any migration into space anytime soon. This is a case of a 7 year old boy watching Star Trek and believing it might be something like this when I got as old as Bill Shatner was…..is.
If you trouble shoot the problem and use a decision tree to find solutions from a survey of available technology it becomes clear that the contender technologies are either available or readily available in the form of improved copies of half century old prototypes.
If you want to launch a program of under ice manned submarine expeditions to the outer planet moons you would have to start with a huge program of super heavy lift vehicles. This may sound expensive but unlike any other existing approach this single inflexible path has the potential of a hundred thousand fold return on investment.
By landing payloads at the lunar polar ice deposits, the exploitation of Lunar Solar Power resources can begin. I want Greenpeace, and every environmental conservation group that exists to understand that they can have there every dream come true if they ruthlessly exploit the solar energy striking the Moon’s surface. It should be understood that the Moon is dead. It is kind of like Nietsche saying God is dead and God then saying Nietsche is dead; do not worry about polluting the Moon- it is dead.
Once there is Lunar Solar Power to melt ore then large structures can be created. These gargantuan solar energy devices will rapidly cover vast swathes of the lunar surface. The more energy that pours into the metal shops the more energy will be produced for new metal shops. After the energy is available come the microwave transmitting antennae fields. Finally come the space power relay battle stations; these must launch to Earth geostationary orbit. Ever larger battle stations from the Moon will park themselves above a certain spot on the Earth where receiving dishes the size of entire valleys are being built. At some point done the road a river of clean electricity begins to flow from the Moon to Earth.
This rainbow bridge of microwave energy from the surface all the way up to geostationary orbit means powering a beam propulsion launch system of several thousand Isp with power all the way from the Moon. How quickly this airline to space begins operation depends entirely on the resources put into it. Exactly what proportion of this electricity shall be dedicated to establishing new worlds in space?
Though it seems impossible for us to imagine, after every single human being on Earth has a very high western standard of living, what will we do with the excess power? If that Lunar Solar Power is geometrically increasing and feeding itself by adding ever increasing and phenomenal amounts of electricity will it be shut down at that point?
I believe when the full of potential of space is explained to the public and they are convinced then no one will stop a geometric progression of Lunar Solar Power to facilitate new projects. The next big project will be constructing large structures in space at the nearest Libration point. Proposed in 1929 the Bernal Sphere may be a simple structure to melt with microwaves in zero gravity and form into a miles- in-diameter hollow sphere. How many miles in diameter is the question.
When the optimum form and thicknesses of the shell or shells-inside-the shells is determined this will determine the diameter of the sphere. Spheres 20 or 30 miles in diameter have the prospect of being a multi-purpose vehicle; Not only can they act as power relay stations but they can support populations of at least thousands in deep space. Propelled by H-bombs this first product of the industry of the Moon may fill many roles from the start a chain of these hollow moons will stretch out in solar orbit from the Earth.
Human beings have a lot of problem with scale. It takes a certain open mindedness to embrace construction projects of such vast scales. But we have only to look at the pyramids in Egypt to understand that large goals have been met since civilization has been in existence. The numbers tell us this chain of hollow moons can contain larger and larger populations as they grow in sized. If it is actually possible to feed materials into a solar furnace and blow up balloons several miles in diameter then the next factory to build may be around Mercury. Greenpeace may have trouble getting tourists to visit the abandoned Earth.
If all else fails, there is sports. Decades ago I read a short story, called “Moonball” I think, and it had professional sports played in superdomes under the Lunar Surface and televised to Earth.
We could have only females of chilbearing age allowed on the Moon and have them play sports. So if there is an engineered pathogen or the planet killer comet or asteroid hits or a super-volcano lights off the next ice age, we will always have our amazon women on the Moon to save our species.

Weinbaum has a Titan that’s more chilly than frigid, one in which the characters flee native lifeforms as they make a forced march over Titanian mountains, all the while carrying the objects of the journey from Earth, the rare gems called ‘flame orchids.’

If we are talking old SF about Titan, then we shouldn’t forget E.E. “Doc” Smith’s Spacehounds of the IPC– which you can read here at Project Gutenberg.

Smith’s extravagant tale begins with an interplanetary spaceship on its way to Mars coming under attack by a mysterious round spacecraft, which slices the hapless terrestrial spacecraft into slices and drags them to the outer Solar System. Two human survivors manage to escape on a wedge-shaped chunk and make it into a makeshift spaceship, land on the moon Ganymede (here imagined as a jungle moon populated by carnivorous plants and belligerent six-limbed natives!!), but are eventually forced to flee.

Later, they encounter a Titanian spaceship that has come to explore Jupiter, which destroys the sinister spherical craft that pursues our terrestrial heros, and invites them to come to Titan. Smith imagines Titan has a place of extreme, frigid cold, where the native humans have adapted to use an entirely different bodily chemistry that functions at those temperatures. To the Titanians, the humans are white-hot monstrosities whose blood runs with molten rock, since ice on Titan is frozen as hard as rock and never melts, but they are cordial despite the massive differences in physiology and habitat between Titanians and Terrans.

Spacehounds of the IPC is noteworthy for another reason- the human spaceships apparently use some manner of beamed energy propulsion, involving mysterious “Roeser’s Rays” that beam energy and momentum to receptors on spaceships by means of light beams. This propulsion technology replaced rockets in Smith’s story, after humans reached Mars and Venus in rocket ships and gained new knowledge of physics from Martian scientists. In the words of the main characters-

“With this introduction I can get down to fundamentals. Molecules are particles of the first order, and vibrations of the first order include sound, light, heat, electricity, radio, and so on. Second order, atoms—extremely short vibrations, such as hard X-rays. Third order, electrons and protons, with their accompanying Millikan, or cosmic, rays. Fourth order, sub-electrons and sub-protons. These, in the material aspect, are supposed to be the particles of the fourth order, and in the energy aspect they are known as Roeser’s Rays. That is, these fourth-order rays and particles seem to partake of the nature of both energy and matter….

…energy can be converted into something that acts like matter through Roeser’s Rays, and that is the empirical fact underlying the drive of our space-ships, as well as that of almost all other vehicles on all three planets. Power is generated by the great waterfalls of Tellus and Venus—water’s mighty scarce on Mars, of course, so most of our plants there use fuel—and is transmitted on light beams, by means of powerful fields of force to the receptors, wherever they may be. The individual transmitting fields and receptors are really simply matched-frequency units, each matching the electrical characteristics of some particular and unique beam of force. This beam is composed of Roeser’s Rays, in their energy aspect…

…this whole thing of beam transmission is pretty crude yet and is bound to change a lot before long. There is so much loss that when we get more than a few hundred million kilometers away from a power-plant we lose reception entirely. But to get going again, the receptors receive the beam and from them the power is sent to the accumulators, where it is stored. These accumulators are an outgrowth of the storage battery…

…From the accumulators, then, the power is fed to the converters, each of which is backed by a projector. The converters simply change the aspect of the rays, from the energy aspect to the material aspect. As soon as this is done, the highly-charged particles—or whatever they are—thus formed are repelled by the terrific stationary force maintained in the projector backing the converter. Each particle departs with a velocity supposed to be that of light, and the recoil upon the projector drives the vessel, or car, or whatever it is attached to. Still with me?”

“Struggling a little, but my nose is still above the surface. These particles, being so infinitesimally small that they cannot even be detected, go right through any substance without any effect—they are not even harmful.”

“Exactly. Now we are in position to go ahead with the lights, detectors, and so on. The energy aspect of the rays you can best understand as simply a vibration in the ether—an extremely high frequency one. While not rigidly scientific, that is close enough for you and me. Nobody knows what the stuff really is, and it cannot be explained or demonstrated by any model or concept in three-dimensional space. Its physical-mathematical interpretation, the only way in which it can be grasped at all, requires sixteen coordinates in four dimensions, and I don’t suppose you’d care to go into that.”

E.E. “Doc” Smith’s explanation states that power generated by power stations back on Venus, Earth, and Mars are directed as mysterious rays to receptors on spaceships, and then is stored and converted into high-momentum streams of particles that provide recoil forces for propulsion. Smith explores the limitations of such a system- the spaceships are limited in how far they can travel by the limited range (over interplanetary distances) of the transmitter system, which leaves our protagonists stranded in the outer solar system. The power transmission system is, in Smith’s story, the alternative to power plants using intra-atomic energy (as are used in his Skylark series), which the characters suspect to be totally impossible.

This is the earliest instance of beamed-energy spacecraft propulsion I have found in SF, and unless someone knows of any earlier use, the craft in Spacehounds of the IPC are the earliest visionary SF spacecraft to use a propulsion system that leaves the heavy parts of a rocket at home- the energy supply and propellent- and rely on energy and momentum “beamed” to the spaceship through some sort of power transmission system. Since they could not carry the heavy power plants with them, the characters transmitted the necessary propulsive energy and momentum to their spacecraft with mysterious rays- which is exactly the same sort of thinking that underlies Forward’s laser pushed lightsails, the Starwisp, and the concepts for laser-electric rockets and ramjets. Leave the heavy power supplies etc. at home, where mass and energy is readily available, and beam energy to a spacecraft so it is not limited to fuel it has carried along with it. The main (science fictional) difference between Smith’s imaginary “Roeser’s Rays” and real beamed energy concepts is that Roeser’s Rays can somehow be converted into high-momentum particles that a spaceship’s driving projectors use to thrust in any direction, rather than being limited to bouncing photons off sails or energizing stored propellent like the real (not fictional) concepts.

If E.E. “Doc” Smith really did explore the idea of beamed-energy spaceships first, those interested in interstellar travel should take note of it- several starship propulsion concepts suggest leaving the heavy parts of the engine at home and beaming energy to a spaceship across great distances. I’ve seen a lot of rocket ships and mysterious “inertial drives” and “warp drives” in early SF, but never a spaceship that rode a beam until Spacehounds of the IPC, and this space opera tale could be quite significant for that fact. Any thoughts?

Christopher, fascinating, and I can’t think of anything earlier by way of beamed propulsion. I know Adam Crowl has thoughts on Spacehounds of the IPC as an early instance of writing about the concept — he may want to wade in here, as he’s written the reference up in an upcoming paper. This 1931 story precedes the discovery of the laser by almost thirty years. Nice catch!

Indeed. Chris has spotted one of the earliest beamed propulsion concepts I’m aware of. I suspect Doc Smith was knowingly fudging the energy requirements – he would’ve been aware of the 300 MW photon power needed for every measly Newton of thrust, as well as the low energy density of accumulators (batteries) in his day. But conceptually the idea has merit.

Another fun part of Smith’s tale is comparing the original 1931 text with the novelisation from the 1940s. In the 1931 version Goddard’s rocketry efforts get Roeser to Mars, while mention of Goddard gets dropped post Goddard’s death in 1945. There are other subtle updates throughout.

Wow, thanks!! By the way, Smith’s story is actually titled Spacehounds of IPC – no “the”, slight mistake there. (: This is the earliest story I could find on the concept of beamed power- E.E. “Doc” Smith did have a fascination with rays of many different and mysterious sorts, so it shouldn’t be surprising that he imagined applying them to spaceship propulsion as well as weapons etc.

In some ways, I think Smith’s Spacehounds of IPC is more believable than some of his earlier work, like the Skylark series- the travel is purely interplanetary, and he lays down a plausible development of space propulsion from Goddard’s first liquid fuel rockets (in the backstory, Goddard sent a rocket to the Moon), through ever more powerful rocket ships capable of traveling to Mars, and then to non-rocket beamed energy propulsion where the energy is generated back at home and “beamed” to the spacecraft.

I think it is also significant that in Spacehounds of IPC, Smith portrayed interplanetary travel as a risky, technically challenging activity that took organizations much larger than one individual decades to make routine. This is in marked contrast to the plot of the Skylark series, where a single brilliant chemist discovers the secret of releasing intra-atomic energy and builds an FTL spaceship in secret with the help of a wealthy friend.

Some SF writers seem to have a hard time with the sheer scale of space travel, and the size of the organizations it takes to achieve it. A couple can’t throw some supplies in a wagon and go settle another planet, and a single Edison-like inventor can’t solve all the problems of space travel by himself. But, in Spacehounds, Smith abandons the “lonely inventor” myth and instead portrays routine interplanetary flight as requiring the mental effort of many scientists, both human and alien, and involving a beamed power system built on on all three inhabited inner solar system planets.

As starships will undoubtedly not spring fully formed from the mind of one brilliant person, but will require the contribution of many people in every area from propulsion, navigation, and communication through life support systems etc., this is an important development- a space opera that more realistically portrays the scale of the effort required to build spaceships.

When we head for the stars, the scale of the enterprise will likely be large, possibly involving development of solar system resources and power stations located near the Sun for beamed power systems- much like we see in Spacehounds, only on an even larger scale…

Since Spacehounds of IPC published in 1947 predates Dan Dare’s first publication in Eagle in 1950, E E Doc Smith has precedence. I wonder if Hampson had read Smith before defining how Spacefleet’s ships worked?

GaryChurch, that’s a wonderful vision of the possibilities still available within our lifetimes. I think there are great problems with one element of it though.

When I tried to argue for the utility of power satellites in the far future, Eniac had a fair bit to say a few months back. Transmission problems seemed the worst of it. The relevant part come near the end of the comments here…https://www.centauri-dreams.org/?p=21719

Hahaha, I loved ‘Spacehounds…’ to bits when I was a kid, but if I remember correctly Smith’s gender relationship dialogue in that book, as with all of this others, was written with far less aplomb than his future-tech speculations :).

I don’t think titanium is ideal here, at all. Glass is quite a bit stronger in compression, and the ultimate would probably be diamond, which will also become a lot less pricey in the next decade or two. Both have the additional advantage of being transparent and thus affording a view.

“Glass is quite a bit stronger in compression”
Yes, but we do not know how to build a glass pressure sphere for a very deep diving submarine while we do know how to build one out titanium. And there is titanium on the Moon. Just working with what I have on this backward primitive planet.

“I think there are great problems with one element of it though.
When I tried to argue for the utility of power satellites in the far future, Eniac had a fair bit to say a few months back. Transmission problems seemed the worst of it. ”

Kevin Parkin is using gyrotrons- a piece of technology originally developed for clean fusion energy. The technical solution is a large recieving antennae; as in miles and miles of antennae field. If all else fails, big somtimes works.

The technical solution is a large recieving antennae; as in miles and miles of antennae field. If all else fails, big somtimes works.

Right, like large fields of solar panels, which would produce similar amounts of power as those receiving antennae. From a source that is already in place, free, and good for billions of years of maintenance-free operation.

Gyrotrons are great, but let me point out that current gyrotrons have an efficiency of around 30%[1], which is not as good as you would like for a power transmission system. Cooling requirements would be enormous, not easy to provide in space or on an airless body like the moon.

I scanned the link real quick and saw 36%. Thanks for that link by the way.

If you want to take a look at what it takes to make coal, gas, nuclear, and solar come out of your wall plug, you might realize that river of electricity pouring down from heaven comes with no strings attached. This makes lower numbers compared to other processess less meaningul.

Cooling requirements are not going to be a problem on Earth or the Moon, but that critical relay station in geosynchronous orbit may be problematic and mess up the whole concept. The devil is in the details.

If you want to take a look at what it takes to make coal, gas, nuclear, and solar come out of your wall plug, you might realize that river of electricity pouring down from heaven comes with no strings attached.

I took a look. Fossil fuel power plants are up to 60% efficient[1], and transmission is at ~93%[2]. A well designed grid could therefore reach 50% end-to-end. That is already better than the gyrotron efficiency alone. Once you add photovoltaic efficiency, beam losses, and rectenna efficiency, I would be surprised if you could reach any better than 10%, for a single stage. Add a relay stage, and you are down to 2-3%.

that river of electricity pouring down from heaven comes with no strings attached

I am not sure what you mean by “strings”, but having to develop a complete industry able to produce photovoltaics on the moon is an incredibly tall order. So is importing them from Earth.

If you are concerned about environmental or safety issues, terawatts of microwaves directed at Earth is likely to raise some eyebrows, or worse.

A note on those 7% transmission line losses. Transmission line losses are mostly proportional to the distance between producer and consumer, with some “lumps” due to transformers, AC/DC conversion (for DC lines), etc. Existing system averages are just that — an average — not a planning number.

This is sometimes a public policy dilemma. For example, it is better to put a nuclear or coal generating plant close to major population centers to reduce losses, but there are obvious trade-offs with safety or perception of safety. High transmission losses are inherent in some production methods, such as hydro-electric where the plants must typically be far from the consumer.

If a microwave receiving array is placed in a desert, the transmission losses to consumers would be high. I would expect that the relevant economics of this have been factored in the economic viability of microwave power beaming from space to ground.

Cooling requirements are not going to be a problem on Earth or the Moon

I wouldn’t lump the two together. The most favored way of cooling a power plant on Earth is to literally let a river flow through it. On the moon? Got to think of some other way…

“Not a problem” is certainly not the right way to describe it.

The easiest would be to operate the devices hot and cool them radiatively. That would work in space and on the moon. Unfortunately, as far as I know, gyrotrons, klystrons, and most other favored sources of microwaves have permanent magnets in them, which would limit their operating temperature. At 36% efficiency, the required radiator surface may well exceed that of the solar collectors.

On the moon, you might build a giant reverse geothermal heat exchanger by burying miles and miles of pipes into the ground. This will be at least as costly as erecting the solar collectors, and come with all the trappings of hydraulic systems, leaks foremost among them.

If you are thinking about the ice said to be at the poles, that would not really be of much use for cooling, either. The ice would have to be mined and brought to the plant, similar to how coal on Earth is mined and brought to the plant. The amounts needed would likely be similar, too. In any case, there are probably better things you are planning do with this precious limited resource than evaporate it in a power transmission plant.

From a distance, most of the Saturnian moon Dione resembles a bland cueball. Thanks to close-up images of a 500-mile-long (800-kilometer-long) mountain on the moon from NASA’s Cassini spacecraft, scientists have found more evidence for the idea that Dione was likely active in the past. It could still be active now.

“A picture is emerging that suggests Dione could be a fossil of the wondrous activity Cassini discovered spraying from Saturn’s geyser moon Enceladus or perhaps a weaker copycat Enceladus,” said Bonnie Buratti of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who leads the Cassini science team that studies icy satellites. “There may turn out to be many more active worlds with water out there than we previously thought.”

Other bodies in the solar system thought to have a subsurface ocean – including Saturn’s moons Enceladus and Titan and Jupiter’s moon Europa – are among the most geologically active worlds in our solar system. They have been intriguing targets for geologists and scientists looking for the building blocks of life elsewhere in the solar system. The presence of a subsurface ocean at Dione would boost the astrobiological potential of this once-boring iceball.

Hints of Dione’s activity have recently come from Cassini, which has been exploring the Saturn system since 2004. The spacecraft’s magnetometer has detected a faint particle stream coming from the moon, and images showed evidence for a possible liquid or slushy layer under its rock-hard ice crust. Other Cassini images have also revealed ancient, inactive fractures at Dione similar to those seen at Enceladus that currently spray water ice and organic particles.

The mountain examined in the latest paper — published in March in the journal Icarus — is called Janiculum Dorsa and ranges in height from about 0.6 to 1.2 miles (1 to 2 kilometers). The moon’s crust appears to pucker under this mountain as much as about 0.3 mile (0.5 kilometer).

“The bending of the crust under Janiculum Dorsa suggests the icy crust was warm, and the best way to get that heat is if Dione had a subsurface ocean when the ridge formed,” said Noah Hammond, the paper’s lead author, who is based at Brown University, Providence, R.I.

Dione gets heated up by being stretched and squeezed as it gets closer to and farther from Saturn in its orbit. With an icy crust that can slide around independently of the moon’s core, the gravitational pulls of Saturn get exaggerated and create 10 times more heat, Hammond explained. Other possible explanations, such as a local hotspot or a wild orbit, seemed unlikely.

Scientists are still trying to figure out why Enceladus became so active while Dione just seems to have sputtered along. Perhaps the tidal forces were stronger on Enceladus, or maybe the larger fraction of rock in the core of Enceladus provided more radioactive heating from heavy elements. In any case, liquid subsurface oceans seem to be common on these once-boring icy satellites, fueling the hope that other icy worlds soon to be explored – like the dwarf planets Ceres and Pluto – could have oceans underneath their crusts. NASA’s Dawn and New Horizons missions reach those dwarf planets in 2015.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., a division of the California Institute of Technology, Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate in Washington. JPL designed, developed and assembled the Cassini orbiter and its two onboard cameras. The imaging team consists of scientists from the United States, England, France and Germany. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

Hammond’s work was funded through a NASA Outer Planets Research grant.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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